As you know, atoms are extremely small. With how small they are, and how erratically they can behave, it’s usually only useful for us to discuss them interacting in very large quantities. This is one of many reasons why we use the mole, which is a measure of quantity.

In order to understand how we convert between atoms and moles, let’s begin by going over a few vocabulary words.

First, Avogadro’s number, abbreviated N-sub-A. This is just a number with no units. It’s equal to approximately 6.022 times 10 to the 23rd. It is hard to express just how huge this number is. If you had Avogadro’s number of tennis balls, that would be enough tennis balls to cover the entire Earth 10 million times over. This is a seriously huge number. Avogadro’s number is critical in chemistry, because it is how many atomic mass units are in 1 gram. This is going to be our conversion factor in a lot of calculations in chemistry.

Lastly, is the unit called the mole, which is weakly abbreviated by just dropping the “e.” Lazy, right? The mole is a measure of quantity. When you have Avogadro’s number of something, you say you have 1 mole of it. 1 mole of water is 6.022 times 10 to the 23rd water molecules. A mole of helium is 6.022 times 10 to the 23rd helium atoms. We can use this to talk about atoms, molecules, ions, whatever. Moles are how we convert between mass and quantity of atoms in a reaction.

For example, if we have 1.5 moles of CO2, how many molecules is that? Well, we multiply this value by a conversion factor that cancels out moles. That will be 6.022 times 10 to the 23rd molecules of CO2 over 1 mole of CO2, because those are equivalent values. Moles cancel with moles, and multiplying this out gives 9.033 times 10 to the 23rd molecules of CO2. Looking at this number, you can see why we usually like to stick to just talking about moles.